Neutrino Mixing and Oscillations in 1999 and beyond A

The interpretation of the existing experimental eveidences for oscillations of neu-trinos in schemes with three and four neutrino mixing is reviewed. Forms of the lepton mixing matrix allowed by the neutrino oscillation data are considered. The possible neutrino mass spectra compatible with the observations are analyzed. The possibility to obtain information about the neutrino mass spectrum from the future 3 H β−decay and (ββ) 0ν −decay experiments is also considered.